In recent years, several kinds of mobile communication systems providing email services to users of mobile stations, such as mobile phones, which are connected to the Internet through various mobile communication networks, have come into use. In the mobile communication systems, emails addressed to subscribers of mobile stations are at first received by email servers, who in turn deliver the emails to the subscribers of mobile stations.
Generally speaking, in a mobile communication network, the communication status of a mobile station is unstable, and can cause communication failures. Therefore, when emails are delivered during a communication failure, a mobile station is unable to carryout the routine function of receiving the emails, which would produce an increase of communication traffic from the large number of undelivered mail leading to a traffic congestion, especially on narrowband networks such as mobile packet communication networks.